1. Field of the Invention
The present invention relates to an engine control method and apparatus designed to allow a compression self-ignition combustion to be performed in at least a part of an engine operating region.
2. Description of the Background Art
Heretofore, in a spark-ignition gasoline engine having a spark plug, a method has been implemented which is designed to perform a compression self-ignition combustion based on self-ignition of an air-fuel mixture, in a partial engine load operating region, and to perform a spark-ignition (SI) combustion based on forced ignition of an air-fuel mixture by a spark plug, in another operating region on a higher engine load side than the partial engine load operating region, as disclosed in JP 2007-292060A.
As for the method designed to selectively use the compression self-ignition combustion and the spark-ignition (SI) combustion depending on engine load in the above manner, various techniques have heretofore been proposed. Specifically, the compression self-ignition combustion is a combustion mode where an air-fuel mixture self-ignites simultaneously at many positions in a combustion chamber, and considered to have a potential to provide higher efficiency than the commonly-used spark-ignition combustion. However, the compression self-ignition combustion has a problem with combustion controllability (i.e., preignition or knocking is more likely to occur) when the engine load is high, so that it is necessary to perform the spark-ignition combustion with high controllability, in a high engine load range. For this purpose, the two combustion modes, i.e., the compression self-ignition combustion and the spark-ignition combustion, are selectively used depending on engine load. This allows the compression self-ignition combustion to be adequately performed, but partially, so that engine thermal efficiency is enhanced in an engine operating region subject to the compression self-ignition combustion, which provides an advantage of being able to improve fuel economy performance.
In the engine disclosed in the JP 2007-292060A, the compression self-ignition combustion and the spark-ignition combustion are selectively used depending on engine speed as well as engine load. Specifically, the engine is designed to perform the compression self-ignition combustion primarily in a low engine speed range, and switch the compression self-ignition combustion to the spark-ignition combustion, in the remaining engine speed range. The reason for switching to the spark-ignition combustion in a high engine speed range is that, when the engine speed is high, a period where an air-fuel mixture is exposed to high temperatures and high pressures (high-temperature/high-pressure period) becomes relatively short, so that the air-fuel mixture is less likely to self-ignite, and misfire is more likely to occurs.
Meanwhile, it is known that, as means for enhancing the engine thermal efficiency, a technique may be employed which is designed to burn an air-fuel mixture under an air-fuel ratio leaner than a stoichiometric air-fuel ratio. Specifically, when air is introduced into the cylinder in an excess amount with respect to a fuel (gasoline) supply amount to perform combustion under a resulting lean air-fuel ratio, a combustion temperature can be lowered as compared with combustion under the stoichiometric air-fuel ratio, so that an exhaust loss and a cooling loss of the engine are reduced, which makes it possible to further enhance the engine thermal efficiency.
Therefore, if the compression self-ignition combustion can be performed under the condition that the air-fuel ratio is set to a lean value, it is expected to make it possible to more effectively enhance the engine thermal efficiency so as to improve the fuel economy performance.
However, it is assumed that, if the air-fuel ratio is simply enleaned, self-ignitability of an air-fuel mixture will deteriorate, and thereby misfire is more likely to occur when switched to the compression self-ignition combustion. Consequently, an engine speed range which allows for the compression self-ignition combustion is narrowed, and thereby the effect of enhancing the engine thermal efficiency deteriorates. Thus, there is a need for a technique capable of allowing the compression self-ignition combustion to be performed in a wider engine speed range, even under an enleaned air-fuel ratio.